Inline inter-core phase control in multi-core fiber with a commercial polarization controller

Inter-core phase control is a prerequisite for interferometry in multi-core fiber (MCF), but no compact inline element for this purpose is presently available, and existing approaches require fanout to single-mode fiber or free-space optics. We show that a commercial inline polarization controller, a low-loss and widely available component, can be operated as an inline inter-core phase shifter. Two-core interferometry on a jacketed four-core patchcord gives a reversible inter-core phase of $0.95 \pm 0.02$~rad from a single device, with an operating-point-independent lower bound of $0.38$~rad, driven through repeated load-unload cycles at a rate set by the actuator. A diametrically compressed disk model is used to predict the operating condition: with the input polarization aligned to a principal stress axis the device imparts a relative phase between cores without rotating the polarization state of either, which we verify by polarimetry on all four cores. The response is polarization dependent, falling to less than $0.58$~rad for the orthogonal input. The result is a practical, inline route to inter-core phase control and stabilization in all-MCF interferometers, requiring neither fanout to single-mode fiber nor free-space optics.